This invention relates to a system for use in carrying out communication between a plurality of stations through an electric power line, such as a power transmission line, a distribution line, or the like.
As a rule, an electric power line of the type described serves to deliver electric power of a commercial frequency to a wide variety of loads connected thereto. The electric power falls within a commercial frequency band. Various attempts have been made to transmit an information signal between stations through such an electric power line. In this case, it is to be noted that the electric power line is put in bad circumstances for transmission of the information signal because noises, such as a corona noise and the like, or undesired signals, such as harmonic waves of a commercial frequency, inevitably appear on the electric power line. In addition, a variation of the loads gives rise to a variation of a noise characteristic and a transmission characteristic of the electric power line. Anyway, the characteristics of the electric power line are widely variable with time.
Furthermore, transmission of the information signal should not adversely affect any other systems or devices coupled to the electric power line. A limited electric power is therefore shared with transmission of the information signal and degrades a quality of the transmission.
In a paper contributed by Michell Lee to IEEE Transactions on Consumer Electronics, Vol. CE-28, No. 3, August 1983, pages 409-413, and titled "A NEW CARRIER CURRENT TRANSCEIVER I.C.," a conventional system is disclosed which comprises a transmitter for carrying out FSK modulation to transmit a modulated signal conveying an information signal to an A.C. line, namely, an electric power line and a receiver for demodulating the modulated signal by the use of a phase lock loop. The modulated signal falls within a specific frequency band which is different from that of the electric power.
Both of the transmitter and the receiver might be incorporated into a station coupled to the electric power line. In this case, a plurality of stations may be connected to the electric power line to carry out communication therebetween.
As will later be described in conjunction with a few figures of the accompanying drawing, the modulated signal is undesiredly deteriorated by noises because the noises may fall within the specific frequency band. In addition, the characteristics may vary in the specific frequency band.